1. Field of the Invention
The present invention relates to a photoelectric conversion device for converting incident light into an electrical signal and an image reading apparatus using the photoelectric conversion device, such as a facsimile machine or an image scanner.
2. Description of the Related Art
A contact type linear image sensor is used for an image reading apparatus such as a facsimile machine or an image scanner (see, for example, JP 2004-282716 A). FIG. 4 is a circuit diagram showing a photoelectric conversion block 15 of a photoelectric conversion device used for the image sensor.
The photoelectric conversion block 15 includes a photodiode 1, a reset switch 2 for resetting the photodiode 1 to a reset potential Vrst, an amplifier 3 for amplifying an output voltage of the photodiode 1, a transfer switch 4 for transferring an output voltage of the amplifier 3 to a capacitor 5, a reading MOS transistor 6, and a channel selection switch 7.
The photoelectric conversion block 15 shown in FIG. 4 corresponds to one of a plurality of photoelectric conversion blocks provided in the photoelectric conversion device. The photoelectric conversion block 15 is provided for each pixel and connected with a common signal line through the channel selection switch 7 thereof.
The photoelectric conversion device is driven as follows to perform image reading.
When the reset switch 2 is turned on in response to a reset signal ΦR, a cathode of the photodiode 1 is reset to the reset potential Vrst. At this time, the reset potential Vrst obtained before light receiving is amplified by the amplifier 3. Then, when the transfer switch 4 is turned on, the reset potential is stored in the capacitor 5.
After the reset potential Vrst is stored in the capacitor 5, the transfer switch 4 is turned off.
When the photodiode 1 which is reset to the reset potential Vrst receives light, a potential of the cathode thereof is reduced corresponding to the amount of received light.
After the light is received by the photodiode 1 for a predetermined period, the channel selection switch 7 is turned on to read, into the common signal line, the reset potential Vrst which is obtained before light receiving and stored in the capacitor 5. Then, the transfer switch 4 is turned on to read, into the common signal line, a potential of the cathode of the photodiode 1 which is obtained after light receiving.
Therefore, a difference between the reset potential Vrst obtained before light receiving and the potential of the cathode of the photodiode 1 which is obtained after light receiving is detected to perform image reading.
However, the conventional image sensor has a problem that an image cannot be accurately read at the time of start of image reading by a phenomenon as described above.
FIG. 5 is a timing chart showing driving signals of the conventional photoelectric conversion device. When an activation operation is performed while a power source is turned on, the image sensor performs an image reading operation in response to a start signal ΦSTR.
At this time, it is necessary to start reading after the cathode (node C) of the photodiode 1 shown in FIG. 4 is reset to the reset potential Vrst.
The node C is floating in a standby state after the power source is turned on. For example, in the case of a P-type substrate, a potential of the node C becomes substantially a substrate potential VSS by a leak from the photodiode. Therefore, as shown in FIG. 5, it is necessary to execute an idle cycle approximately ten times in order to sufficiently reset a potential of the node C to reset potential Vrst.
Here, a light receiving surface of the photoelectric conversion device cannot be sealed with a resin in the view of the structure of the image sensor, so adhesion of foreign matters on the light receiving surface cannot be prevented.
A conductive foreign matter 8 adhered to the light receiving surface is expressed by an equivalent circuit as shown in FIG. 4. In the standby state after the power source is turned on, a potential of a node D becomes a potential equal to the potential of the node C, that is, substantially the substrate potential VSS.
Therefore, as shown in FIG. 5, the potential of the node D becomes the reset potential Vrst simultaneously with the time when the potential of the node C is reset to the reset potential Vrst. After a lapse of several seconds, the potential of the node D is stabilized at the substrate potential VSS.
Thus, there is a problem that, during a reading period, the potential of the node C is influenced for several seconds in which the potential of the node D is stabilized at the substrate potential VSS, thereby causing an afterimage phenomenon.
FIG. 6 shows the afterimage phenomenon in the conventional photoelectric conversion device. Data obtained by reading a black and white document as shown in FIG. 6 using an image sensor 20 is a read image. A state in which a conductive foreign matter 21 is adhered to a photoelectric conversion element surface of the image sensor 20 is shown in FIG. 6.
Instead of an image of black data, an afterimage of white data immediately before the black data, is produced by the influence of a conductive foreign matter 21 adhered to the photoelectric conversion element surface of the image sensor 20. That is, the afterimage phenomenon occurs.